Are you talking about a sequential numbering system to identify each individual page? A bit like cloakroom tickets? If so, the automatic page number tool will do the trick, even if you want to start from 100. Here's how:
1. Go to you master page(s) and create a text box where you want the numbers to appear. Go to TYPE > INSERT SPECIAL CHARACTERS > AUTO PAGE NUMBER. On the master page, this will show as "A" in the text box.
2. Return to your first page. Go to LAYOUT > NUMBERING AND SECTION OPTIONS. Click "Start Page Numbering At" and insert the number you want to start from (100). Click OK and hey presto!
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(ignore my "Vendor" status... I clicked the wrong thing when I signed up and can't figure out how to change it!)